First post here so apologies if I get it wrong.
Following is an attempt at an adjustable rocker jig for vac-bag board building along the lines of Bert B thread. The idea is the have a fixed rocker from mid point to nose (use wedges for the final nose kick if needed) and have the back half of the jig rocker adjustable for different length boards or more or less tail rocker. Again you can use wedges for any final kicks in the tail.
I put a lot of boards from various shapers into my old EPS jig and found the rockers were all pretty similar except for slight variation in tail rockers/kick.
Materials
2 sheets 3mm MDF 2400 x 600mm
4 strips 6mm MDF 2400 x 100mm + leftovers for jig
1 sheet 25 x 600 x 2400mm VH EPS
1. Mark out rocker profile on 2 sheets of 6mm MDF, don’t put too much nose rocker in cause you can’t bend the 6mm strips too much. Screw spacers (approx. 75mm) between two rocker profiles.
2. With the rocker profile on a flat bench laminate the 4 x 100mm MDF strips over the rocker profile. I placed waxed cooking paper in the middle across the back half of the profile; this stops the 2 halves sticking together in this area. I used std PVA wood glue. Clamp and weigh down the laminate as required.
3. When the laminated ribs have cured remove most of the clamps and using a circular saw with the rip guide set to 50mm slice the rib into two 50mm strips.
4. Using other strips of 50 x 6mm MDF make up end pieces for the ribs. The nose end piece is a 4 strip all glued together, the tail is two pieces of 2 x 50 x 6mm glued together, these are all 600mm long. Glue these to the ends of the laminated ribs. You’ll have to trim 50mm of each end of the ribs so the overall length stays at 2400mm.
5. Now set the 2 rocker profile pieces about 550mm apart with spacers and screw together.
6. On a shaping stand or similar, layout 1 sheet of the 3mm MDF (I also laid out some 6oz, probably overkill) and wet out with epoxy. Place the EPS on top of this (you’ll need to trim 50mm off the perimeter of the original 600 x 2400 EPS sheet, or whatever size gets it to fit between the ribs). Add the rib frame, then place the other sheet of (wet out) 3mm MDF on top of this (i used 6oz again here also). It’s a good idea to have strips of baking paper between the unglued ribs again.
7. Place the MDF and EPS bundle into a vac bag, then weight and clamp the lot over the rocker profile.
8. After its cured and out of the vac bag you’ll probably need to run a knife around the back edges to pull out the baking paper and free it up a bit. The idea is to make sure the MDF ribs haven’t re-stuck together so the back end can flex. In hindsight i would’ve been better not sticking the EPS to the 3mm MDF over the back half of the jig, or i could’ve laminated less of the 6mm MDF ribs together over the back half also. It initially didn’t flex freely enough. I ended up pulling it open to crack the 3mm MDF off the EPS, this helped a bit, but less gluing would probably make it a bit freer also.
9. Using the cabinet fasteners (I had to trim the bolts down a bit), drill a series of holes along the free edges through the MDF side ribs, these will act as clamp once you’ve flexed to the desired rocker. I've got four in each side but more would be better, especially if you had more flex in the tail end of the jig
The idea is you can lay the jig over a board you like, tighten up the fasteners and replicate the rocker. This would work if I'd glued less. As it is now i get about 10mm variation in tail rocker. If your ambitious you could probably have a similar arrangment at the front of the jig to adjust nose rocker, but it's likely to be harder to bend and control that sort of curvature. It's handy also to have a good flat bench to true up the jig before vac-bagging, so far this jig has stayed pretty true.
Any way, over to you.
